#include<bits/stdc++.h>
using namespace std;
const int M = 1000000007;
using P = pair<int, int>;


int main() {
    // cin.tie(0);
    // ios::sync_with_stdio(0);
    int n, l;
    scanf("%d%d", &n, &l);
    l *= 60;
    vector<int> a(n);
    for (int i = 0; i < n; ++i) {
        int x, y;
        scanf("%d:%d", &x, &y);
        a[i] = min(l, x * 60 + y);
    }
    double conv[n];
    for (int i = 0; i < n; ++i) {
        conv[i] = 1;
        for (int j = i - 1; j > 0; --j) {
            conv[j] += conv[j - 1];
        }
    }
    double dp[n][l] = {};
    double dp2[n][l];
    dp[0][0] = 1;
    for (int i = 0; i < n; ++i) {
        for (int j = n - 1; j > 0; --j) {
            for (int k = l - 1; k >= a[i]; --k) {
                dp[j][k] += dp[j - 1][k - a[i]];
            }
        }
    }
    double ans = 0;
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) {
            for (int k = 0; k < l; ++k) {
                dp2[j][k] = dp[j][k];
                if (j > 0 && k >= a[i]) {
                    dp2[j][k] -= dp2[j - 1][k - a[i]];
                }
                ans += dp2[j][k] / conv[j];
            }
        }
    }
    cout << setprecision(16) << ans / n << '\n';
    return 0;
}